1. Field of the Invention
The present invention relates to a termination resistance adjusting circuit for adjusting a termination resistance used in an interface circuit, for example, according to an external resistor.
2. Description of Related Art
For a termination resistor used in an interface circuit, 50Ω±10% of absolute value accuracy is required for the HDMI specification, for example. As the process variation of a resistance of a semiconductor resistor, such as diffusion layer resistor and polysilicon resistor, exceeds 10%, the above specification cannot be satisfied and they cannot be used as a termination resistor.
For the abovementioned problem, Japanese Unexamined Patent Application Publication No. 2003-204247 discloses, in FIG. 3, a variable resistor circuit which can specify a resistance according to an external resistor with high absolute value accuracy of a resistance. If an input or output signal supplied with this specified resistance is a differential signal, the specified resistance could fluctuate by fluctuation of the common voltage of the differential signal. However, Japanese Unexamined Patent Application Publication No. 2003-204247 does not disclose anything about the fluctuation of the common voltage.
Japanese Unexamined Patent Application Publication No. 2002-344300 discloses, in FIG. 4, an impedance adjusting circuit which can adjust a termination resistance according to an external resistor with high absolute value accuracy of resistance, and also keep the termination resistor to be constant even if the common voltage of an input differential signals changes.